Well, MS also have to sell servers. In fact, servers are a much larger business for MS than phones are.
In the past, MS have done very well by supporting only their own platforms (and perhaps another non-threatening platform such as MacOS). That's why MS Office has never appeared for Linux. But the situation on mobile is different. If they limit themselves to supporting only Windows Phone, they may lose sales of servers. Of course, by supporting iOS and Android, they lose a strategic opportunity to promote WP by tying popular products to it, but in the bigger picture and in the interests of their shareholders, this seems like the right move.
Having said that, I am surprised that MS are not supporting this on WP yet.